Analysis of key factors affecting the quality of offset printing

High-quality prints are the ultimate goal of printing. Print quality mainly refers to the comprehensive effect of various characteristics of the printed matter, that is, absolute quality. On the other hand, it must be requested by the customer. The printed matter is consistent with the customer's sample. This faithfulness to the original copy is called relative quality. There are many factors that affect the printing quality. This paper only analyzes the three key factors affecting the printing quality from the printability of paper and ink, pre-press technology and reproduction technology, and explains the internal relationship between them to improve the printing quality.

1. Printability of paper and ink

1. The effect of paper performance on printed matter

Print quality is highly related to the type of paper. Specifically, it is related to the five aspects of paper surface smoothness, ink absorption, strength, PH value and so on.

â‘  The influence of the smoothness of the paper on the printing quality.

The smoothness of the paper is actually expressed as the size of the gap on the surface of the paper. Therefore, the smoothness has a greater impact on the quality of the printed matter. The smoothness of the paper is high, the layout graphics on the printed products can be faithfully reproduced, especially the small dots can be accurately restored. The paper with low smoothness, because of the concave shape on the paper surface, is easy to make the dot printing unclear, and the level of the printing plate is not clear. The smoothness and thickness uniformity of the paper will have a direct impact on whether the printing pressure and ink color can be kept uniform. However, to ensure the smoothness of the paper, mainly adopt methods such as adding filler and calendering, so that the tightness of the paper is increased, and its elasticity and ink absorption are relatively reduced, so the back of the printed matter is prone to smearing. In addition, when the printing pressure is too high, it is easy to spread the ink layer and expand and deform the print. Based on these circumstances, the uneven smoothness of the paper and the obvious difference in the rough light should be classified separately, so that when printing paper with poor smoothness, the printing pressure and the amount of ink are appropriately increased to obtain a clear imprint, thereby improving printing quality.

â‘¡The effect of paper ink absorption on the printing quality.

The transfer of ink is accomplished by the capillary action of pressure and the interweaving gap of paper fibers. That is, when the paper is compressed, the interwoven fiber gap compresses, and after the pressure disappears, the surface of the paper returns to its original shape due to the elasticity and absorbs ink at the same time. This is the process of ink layer formation. Various papers have different elasticity due to the fibrous structure, so the ink absorption is also different. Paper with good ink absorption requires light printing pressure, and the pressure resistance of the printing plate is high. If the paper is too slack, the fiber gap is too large, then when the ink viscosity is large, it will produce paper ills; on the contrary, when the ink viscosity is small, it will easily cause the linking material to quickly penetrate, causing through-printing phenomenon, In this regard, it is necessary to increase the viscosity of the ink to adapt to the printability of the paper. Therefore, the paper with tight paper quality and less ink absorption is mostly used for color printing, which can avoid the pigment falling off after the connection material in the ink penetrates, and the gloss of the ink layer will affect the printing quality, but it is also easy to produce slow drying and easy to stick Dirty, crystallization and other undesirable phenomena.

â‘¢The effect of the PH value of the paper surface on the printing quality.

The role of the polyvalent metal salt in the acid paper and the desiccant generates insoluble oils and fats, which prevents the oxidative film formation of the ink and inhibits the drying of the ink. Therefore, the acid paper should be appropriately increased when printing. Neutral or alkaline paper is good for ink drying. The PH value of the paper surface is high (alkaline), and the ink dries quickly on the paper surface. For offset printing, the acidity and alkalinity of the printed paper will also affect the pH of the fountain solution. As a result, the drying speed of the printing ink is affected, and the printed matter is poorly dried and the backside is dirty and other undesirable phenomena. At the same time, the acidity of the fountain solution is too strong, which will inevitably accelerate the corrosion rate of the printing plate and affect the reproduction of the tone and level of the printed product. When the pH value of the printing paper is too high, that is, the alkalinity is too strong, during the printing process, the alkaline substances in the paper will continue to dissolve and transfer to the fountain solution, and neutralize the weak acidic fountain solution, Changes the pH of the fountain solution. In addition, these alkaline substances will also reduce the interfacial tension on the water-ink interface of the printing plate, promote the emulsification of the ink, cause the non-graphic parts to become dirty, reduce the gloss of the printed product, and make the ink dull. The stronger the acidity of the fountain solution, the greater this effect will be.

â‘£The influence of paper uniformity on printing quality.

For paper, the part that directly absorbs the ink is the paint layer, but when the uniformity of the paper is poor, it will cause the unevenness of the penetration of the adhesive in the paint to the base paper layer, which will naturally cause the paint layer to absorb ink. Uniformity. In addition, the unevenness of the base paper will also cause the unevenness of the coating during the coating process, especially the uniformity of the coating during the blade coating has a great correlation with the uniformity of the base paper, so the unevenness of the base paper Properties often cause unevenness in the ink absorption of the paint layer.

⑤The influence of paper surface strength on printing quality.

The surface strength of the paper is not high, and the phenomenon of fluffing and powdering will occur during printing. These fallouts not only paste the printing plate, but also leave white spots on the paper where ink should be. The imprinted ink film will be uneven or even matt, which seriously affects the quality of the printed matter.

2. The effect of ink performance on printing quality

â‘  The effect of the color performance of the ink on the printing quality.

The color performance of the ink is directly related to the accuracy of the color saturation and hue of the color reproduction of the printed matter. There are four main parameters to measure ink color reproduction:

● The color intensity of the ink can also be called the color density, which is a representation of the color rendering ability of the ink, which affects the neutral gray balance and the accurate reproduction of intermediate and double colors.
● Hue deviation rate is the percentage of the wrong absorption amount of the ink disturbing the basic hue and the corresponding complementary color absorption margin.
● Grayscale is the ratio of the absorption of the ink to the three light areas to the color light that should be completely reflected, that is, the percentage of the maximum reflection density occupied by the ink to the equal absorption area of ​​the spectrum.
● The color rendering efficiency is expressed by the percentage of the actual reflected light of the ink to the ideal reflected light, which is a parameter that comprehensively reflects the size of the ink's selective absorption capacity.

The above four parameters are closely related to the hue, tinting strength, density, fineness, activity, solubility, oil absorption and hiding power of the selected pigment.

â‘¡The influence of the rheological properties of the ink on the printing quality.

The rheological properties of ink mainly include viscosity, viscosity, fineness, and adhesion.

● The greater the viscosity of the ink, the greater the force required to peel the ink layer. Due to the unevenness of ink transfer, if the viscosity of the ink is too large, the surface of the paper is easily peeled off by the ink film during printing, that is, the hair is pulled. For the case where the viscosity of the ink is too large, it is generally adjusted by adding a dilution solvent or mineral oil.
● When the viscosity is too large, the entire ink system is in a supersaturated state, the pigment is unevenly dispersed, and agglomerates are accumulated together, resulting in poor ink transfer effect and fluidity, so that the pigment cannot smoothly enter and exit the mesh, or even not transfer . When the viscosity is too low, it indicates that the ink contains a large amount of organic solvents, and relatively few components such as resins and pigments, so that it cannot form a smooth mask layer during drying, resulting in a dull hue and lack of gloss, that is, the color saturation is reduced.
● Ink imprinting is not true after printing, there are particles, dull gloss, and low printing resistance of the printing plate. The main reason for this phenomenon is that the fineness of the ink is not suitable. The fineness of the ink affects both the quality of the printed product and the durability of the printing plate during the printing process. The higher the fineness of the ink, the clearer and fuller the dots of the printed product, the higher the density of the ink, the stronger the coloring power, and the higher the quality of the print. Conversely, the fineness of the ink is low, the coefficient of friction of printing is large, the ink is unevenly distributed, easy to paste, and the printing plate printing resistance is reduced.
● The adhesion of the ink is too large, and it is easy to appear the phenomenon of fluffing. In the high-speed printing process, some paper fibers are often pulled up, which is the phenomenon of printing and pulling paper wool. The formation of paper fluffing phenomenon is mainly related to the viscosity of the ink and the tightness of the paper.

â‘¢ The effect of ink emulsification on the quality of printing.

The emulsifying properties of the ink mainly affect the water-ink balance in printing. The characteristics of offset printing determine that the ink must have a certain water holding capacity. When the proportion of water and ink changes, the emulsification rate of the ink will also change. When the water supply is small, the fountain solution added to the ink will be fully absorbed, and the emulsification rate will rise rapidly. For actual production, it is hoped that the supply ratio of the amount of water and ink can meet the requirement of keeping the emulsification rate stable, which is convenient for quality control in the printing process. Otherwise, the emulsification of ink will cause changes in ink delivery and distribution, affecting production. The ink and ink balance is just right, the ink can be transferred more ideally, so as to realize the printing effect of dot clean, neat, clear image, rich layers, and colorful colors, and can ensure the accuracy of overprinting, promote the drying of the printed matter, and at the same time ensure the blank Partly clean and tidy. If the water supply and ink volume imbalance of the layout will cause the blank part of the paper to become dirty, the print is dull and dull, and a large range of ink emulsification and other problems, seriously affecting the quality of printing.

â‘£The effect of ink drying performance on printing quality.

Ink drying is not good, there will be sticky dirt. In the printing process, due to the slow drying of the ink, the ink is too soft, too thin or the amount of ink is too large, it will cause the paper graphics to stick dirty or the back rubs. Most of this phenomenon occurs in the field where the amount of ink in the printed matter is large or the density of the layer is high (ie, dark tone). It is generally caused by rubbing dirt and slow drying in the later stage of drying. According to the different generation time, the sticky soil can be generally divided into: the second side embossed sticky soil (that is, when the back surface is printed immediately after one side is printed, the result is that the other side of the picture and text appears on the back) and all the printing is completed and contacted during bundling The ink on the surface is contaminated with each other.

⑤ Ink light resistance.

The color of the printed product will gradually fade under the influence of light. Generally, light-colored inks are more obvious, such as yellow, because the molecular structure of the ink contains more unsaturated bonds, especially the molecular structure of the pigment has changed Caused. Therefore, the light resistance of the ink should be considered when selecting the ink, otherwise the fade of the printed product will affect the quality of the product. [next]

2. Prepress technology

1. The effect of image scanning on printing quality

The most basic job of printing is to reproduce colors, and colors are often expressed through pictures. Image scanning is to convert the color information on the picture into digital signals that can be stored and edited by the scanning device, and then process these digital signals through application software such as image processing software and typesetting software. Therefore, whether the image can be copied accurately and with high quality, that is, as much as possible to retain the tone, gradation, contrast and saturation on the original document, is the basic standard to measure the image scanning quality.

Evaluation of image scanning quality:
â‘  The accurate calibration of the manuscript is the basis of whether the manuscript can be truly restored.
â‘¡ The tone is the hierarchical structure of the whole picture. The complete tone can complete the picture. This is because it is easy to lose the gradation in the darkest and brightest places.
â‘¢When the level of the original is rich, the contrast is strong, and the saturation is high, as long as the level, contrast and saturation of the original can be restored as accurately as possible when scanning.

2. The effect of graphic version on printing quality

The graphic version is to combine the scanned pictures and the entered text according to the layout required by the customer. If the production requirements of the subsequent processes are not fully taken into account during the assembly process, all the printed products may be scrapped. If you do not follow this standard in the assembly version, it may increase the error rate of the post-processes such as cutting and beer boxes, resulting in unnecessary waste. Group version is a quite complicated operation process. In the group version process, attention must be highly concentrated, otherwise a small mistake may cause a large amount of rework.

3. The impact of development on print quality products

â‘  Developer concentration.

If the concentration of the developer is too large and the development is too fast, it is easy to cause excessive development of the printing plate, the lines of the graphics are thin, the fine dots are lost or the graphics are thin and unclear, which affects the printing quality; the concentration of the developer is too small, seeing the surface of the drug film decomposed It is not easy to wash, and it is easy to get dirty when printing on the machine.

â‘¡Development time.

If the development time is too long, the surface of the drug film that is not exposed to light on the printing plate will be easily dissolved, and the graphics of the printing plate will become lighter and thinner, resulting in unrealistic and unclear imprints during printing. The surface is not easy to remove completely, and it is easy to get dirty when printing. The appropriate developing time is that when the printing plate is developed and rinsed, the surface of the drug film decomposed by light is just washed away. If the concentration of the drug solution is relatively large, the development time should be shortened accordingly; otherwise, the development time should be extended accordingly.

4. The effect of output on printing quality

The format produced in the computer needs to be output into a four-color film through a laser imagesetter or directly into a printing plate through a computer plate making machine (CTP), or a digital proofing sample can be output through a digital proofing machine. No matter what the output is, the error rate of this process may be the highest. Several aspects that are prone to errors in the output process:

â‘  Text and pictures.

Text output problems are the most common in the output process. Text garbled characters, text misalignment, text leakage, and font errors often occur. These are mainly caused by fonts, output software, output parameter settings, and output devices. In the output process, the problems that appear in the pictures are mainly wrong pictures and missing pictures.

â‘¡ Output device.

Output devices are also often the main cause of errors. For digital proofers, if the color of the proofs shows a significant deviation, it may indicate that the profile generated during the previous color management is not consistent with the current situation. Therefore, before outputting the file, it must be ensured that the output device is in the best state, and the material used is consistent with the characteristic curve or characteristic file applied by the software.

5. The effect of proofing on printing quality

Proofing plays a role of inheritance, it is very important for printing. It can not only check the previous image scanning, text entry, layout, etc., but also a standard for subsequent printing. Because proofing is analog printing (whether traditional proofing or digital proofing), proofing proofs will definitely be different from the final print, but there is a principle that must be followed is that printing can catch up with proofing. Therefore, the quality of the proofs and the size of the difference with the printed product will have a great impact on printing.

For digital proofing, because the ink used for proofing is different from the ink used for printing, in terms of analog printing, the color space of the proof and the color space of the print are matched by the color management system (CMS) to simulate the printed color of. However, at present, several digital proofing machines that are commonly used are unable to achieve spot color proofing, which is inevitably a major flaw. [next]

Three, reproduction technology

1. The effect of image color deviation on printing quality during printing

When processing an image on a monitor, if the monitor is well calibrated, the color of the image may be relatively normal, but this does not mean that the color is the same after the image is printed, because the monitor uses color light to add color to achieve color Printing uses the color light subtractive method to achieve color, so the color reproduction will be different. In fact, when printing, it is necessary to consider whether the ink used will have a color shift problem, that is, how to control the gray balance of the ink used. Briefly, gray balance refers to a combination of colors that can produce gray colors. Equal amounts of yellow, pin, and cyan do not produce neutral gray. They produce a lighter, more turbid brownish gray, rather than a true gray. The reason for this is the undesirable absorption of colored light by the ink described above, which is caused by the impureness of the ink used.

2. The impact of dot gain on print quality

The dot size is directly related to tone and hue, therefore, increasing the dot will make the whole image darker. Moreover, the dots of different sizes of dots increase into a nonlinear relationship, and the dots formed by the characteristics of paper and ink generally show an exponential expansion law. The other thing to note is that in the process of graphic output of plate making and printing, etc., the dots will also increase due to the factors of materials and equipment.

The dot increase is an inevitable phenomenon due to certain factors in the printing process. In order to truly reproduce the color and gradation of the original, the impact of the dot increase must be compensated during the prepress processing. This compensation process can be applied directly during the graphic processing, or the compensation function can be added to the file first during the processing, and then compensated during the printing and phototypesetting process.

3. The effect of total ink control on printing quality

In theory, the three primary colors of CMY colorants can restore the desired colors. However, in actual printing, K is indispensable. However, when printing with CMYK inks, a lot of ink will accumulate in dark areas. To achieve an ultra-black appearance, such as the cover of books and magazines, only K is Far from being black enough, often need to use C50M50Y50K50, or even C100M100Y100K100, because the amount of ink accumulation must reach at least 200%, or even 400%. Because different substrates have different ability to maintain ink, if no measures are taken to control the amount of ink applied, the dark areas will be pasted, and the dark levels will be lost. Therefore, the total amount of ink needs to be set in the pre-press image processing.

4. The effect of printing register on printing quality

Registration refers to the accurate alignment of two or more printed images with each other. The registration problem is an inevitable error caused by the mechanical factors of the printing equipment, and is also an important aspect in the printability. The registration problem refers to the problem that occurs when the four color separation plates are printed separately. In the printing process, the printed pattern of the latter group is accurately placed on the printed pattern of the previous group. If the registration accuracy is greater than a certain absolute value, it will cause color pattern under-coloring, discoloration, etc. clear. Especially in the production conditions of high-speed online printing, the phenomenon will be more hidden and complicated, and the product quality will be seriously affected.

5. The effect of the color sequence arrangement of the offset press on the printing quality

The color sequence of multi-color offset presses is mostly in inverted color sequence, namely K, C, M, Y. The printing method of the multi-color printer is wet stacking wet, that is to say, the ink is superimposed on another kind of ink in a very short time without the oxidized conjunctiva drying. The color sequence of the multi-color offset printing machine is arranged according to the viscosity of the ink. Put the ink with high viscosity in front of the printing. In this case, the fastness of overprinting will be better, and there will be no "reverse overprinting" due to the problem of viscosity. As a result, the printed picture is blurred, the color is turbid and gray, and the gloss is poor. However, the effect of ink overprinting cannot be entirely attributed to the ink viscosity, but it should be said that the viscosity of the ink and the amount of ink play a role. Therefore, in order to achieve a good overprint effect, the ink should have the following points: the ink must have the appropriate viscosity and concentration, and the wet ink supply should have a moderate amount of ink supply, too much or too little is not good , The ink requires positive hue, high transparency, good gloss, fine ink particles, acid and alkali resistance, and good chemical stability.

6. The effect of printing pressure on printing quality

The printing pressure refers to the interaction force between the plate cylinder and the rubber cylinder, and between the rubber cylinder and the impression cylinder. It is a necessary condition for ink transmission. If the printing pressure is too low, the ink is not easy to transfer, and the ink traces are faint or false. The ink transfer is not good; if the printing pressure is too large, the ink volume expands to the non-graphic part, which makes the dots increase seriously, the image is distorted, and cracks will occur. It is easy to make the printing plate and rubber cloth withstand the pressure drop. Therefore, the mechanical printing pressure must be adjusted reasonably. Different amounts of paper require different printing pressures, and only suitable printing pressures can ensure uniform ink color and consistency.

7. The effect of water-ink balance on printing quality

Ink balance is a relative balance, not an absolute balance. During the high-speed operation of the equipment, the graphic and non-graphic parts of the printing plate are both inked with water and infiltrated with each other, so ink emulsification will inevitably occur on the printing plate. If the amount of water and ink is not well controlled, it will inevitably deepen the emulsification of the ink, resulting in paste and dirty printing. When printing, the method of reducing the amount of water and increasing the amount of ink is appropriate, but some will also reduce the amount of ink. In addition, the ink emulsification will vary with the environment and conditions. In actual production, the ink viscosity and the pH of the fountain solution should be grasped according to the actual situation, and the amount of ink and water should be controlled. The paste version and the dirty version shall prevail.

Chair Set

Chair Set,School Canteen Chairs,School Classroom Chairs,Classroom Sketching Chair

AU-PINY FURNITURE CO., LTD , https://www.aupinyjm.com